You DO NOT disconnect the battery while the engine is running on modern vehicles. You can cause a LOT of problems.
Get a Voltmeter or use the Hidden OBC Menu to verify the running Voltage.
Also keep in mind the E70 and probably other similar platforms have a VERY POOR method to notify the driver there is a charging problem. I really do not think the E70 can notify the driver!!
http://www.scoopz.com/m5board/E60_Hi...structions.pdf
I run a wire UltraGauge on my X5 with threshold alarms for Temperature and Voltage.
UltraGauge OBDII Scan tool & Information Center
Low Voltage an any modern vehicle will cause ALL sorts of crazy problems.